The present invention relates to dry seal gasholders of the type comprising a cylindrical storage tank with a vertically-movable expansion roof or piston. In particular, this invention relates to an improved piston leveling system for such a gasholder.
A typical piston leveling system for a dry seal gasholder is disclosed in U.S. Pat. Nos. 2,504,312, 2,554,765 and 2,579,776 of J. W. Allen and U.S. Pat. No. 2,457,762 of J. H. Wiggins. In this prior art leveling system, one or more weights disposed outside the tank is coupled to the piston portion of the gasholder. Each weight is typically coupled to diametrically opposed points on the piston by two cables, one of which cables is approximately one-half the length of the other cable. In operation, this type of leveling system applies an upward force equal to one-half the leveling weight to each of the opposite sides of the piston to which the weight is coupled. Any change in attitude of the piston is resisted by the leveling weight through an increase in the load applied by the weight to the low side of the piston.
While this prior art arrangement is satisfactory for gasholders up to approximately one hundred feet in diameter, it has been found to be unsatisfactory for larger gasholders. More particularly, the cables are subject to changes in length as a result of day-to-night temperature changes. The longer of the cables will undergo greater thermal changes in length, thereby changing the loading on the two cables and causing the piston to tip.
Furthermore, the prior art leveling systems have not afforded means for conveniently determining the attitude of the piston and the loading on the two cables from outside the tank.